ScopePrinter to exchange printf and make it multithread

ScopePrinter.cpp

#include<string>
#include<iostream>

class ScopePrinter {
  public:
    template<typename STR>
    explicit ScopePrinter(STR&& init, std::ostream& stream = std::cout)
        : stream(stream),
          msg(std::forward<STR>(init)) {
        msg.append(":\t");
    }
    ~ScopePrinter() {
        dump();
    }

    void f(const char *format, ...) {
        va_list ap;
        va_start(ap, format);
        char str[256] = {};
        vsnprintf(str, 256, format, ap);
        va_end(ap);
        this->msg.append(str);
        this->msg.append(" ");
    }

    void dump() {
        {
            std::lock_guard lg(this->streamMutex);
            this->stream << msg << std::endl;
        }
        msg.clear();
    }
  private:
    std::ostream& stream;
    std::string msg;
    std::mutex streamMutex;
};

int main() {
    ScopePrinter print(__FUNCTION__, std::cerr);

    print.f("ptr="%p", &print);
    if(true)
        print.f("always true");
    else
        print.f("still making one line");
}  // <-- will be printed to std::cerr there.
   // std::cerr << "main:  ptr=0x* always true";